1. Field of the Invention
The present invention relates to a method for producing a silicon single crystal through the magnetic Czochralski method, and more particularly, to a method for producing a high-quality silicon single crystal, in which a cusp magnetic field is symmetrically applied to the pulling axis, so as to regulate the oxygen distribution in the single crystal, such that oxygen distribution is uniform with respect to the growth direction, i.e., the axial direction, and to prevent generation of dislocations formed during the crystal growth.
2. Background Art
To a silicon single crystal used in the production of an ultra-large scale integration semiconductor devices, there is applied a gettering technique in which oxygen contained in the single crystal is deposited as an oxide and heavy metal impurities causing deterioration of the yield of the devices are gettered by the oxide contained in the vicinity of a wafer surface. Attaining uniform distribution of oxygen contained in a single crystal is an important factor for attainment of the sufficient gettering performance.
Conventionally, there have been employed several methods in which the rotation rate of a crucible is modified as a silicon crystal grows so as to regulate the oxygen distribution within the crystal. Although these methods achieve a reduction in oxygen concentration as the pulling of the single crystal progresses, generation of high oxygen concentration in a top portion of the crystal immediately after initiation of pulling is difficult to prevent. Therefore, the above methods provide single crystal products that fail to meet standards for oxygen concentration, and thereby disadvantageously deteriorate the yield of the products.
Recently, the standard for oxygen concentration required for a semiconductor device substrate has become more strict corresponding to the enhancement of a functionality of the device, and overcoming the above problems through the above methods has become increasingly difficult. In addition, generation of dislocations in a single crystal, during a pulling step, tends to increase when pulling is carried out in the recently introduced large- scale apparatus for producing a single crystal. In order to cope with the problem of an occurrence of dislocations, a variety of methods involving application of a DC-induced magnetic field to a melt during pulling, have been investigated.
Meanwhile, the effect of an application of a magnetic field on a generation of dislocations in a single crystal during a pulling step is based on suppression of convection in a melt, through application of a magnetic field to the melt. The suppression of convection suppresses incorporation of quartz released from the inner surface of a crucible into the melt, to thereby prevent generation of dislocations in a single crystal during crystal growth (HOSHIKAWA et al., Journal of Applied Physics, 60, 808, 1991). The effect also prevents deterioration of a quartz crucible, to thereby possibly prolong the service life thereof.
Among the methods for applying a magnetic field to a melt, attention has been drawn to a method involving application of an axially-symmetrical and radial cusp magnetic field to a melt in a crucible. According to this method, a pair of magnets through which cicular currents are induced in opposite directions are disposed above and below the melt. As a result, at the position halfway between the two magnetic fields, along the growth axis (hereinafter referred to as "mid-field position"), the magnetic fields cancel each other out to make a vertical magnetic field component zero and to form a radial horizontal magnetic field. The radial cusp magnetic field restrains the flow of the melt, to thereby stabilize the melt. In other words, application of a radial cusp magnetic field induces convection at a portion adjacent to the solid- liquid interface at which crystal growth occurs, and suppresses convection at the remaining portions of the melt, to thereby serve as an effective method for realizing uniform oxygen distribution.
Conventionally, based on the above method involving application of a cusp magnetic field, there have been disclosed a variety of methods for producing a single crystal having a uniform compositional profile. For example, Japanese Patent Application Laid-Open (kokai) No. 5-194077 discloses a magnetic Czochralski method for producing a silicon single crystal, involving application of a cusp magnetic field to a crystal in which the rotation rate of a crucible increases and the intensity of the magnetic field decreases, as the fraction of a solidified silicon melt increases after establishment of the rod diameter of a single crystal.
In the above method, the crystal rod to be pulled and the crucible are rotated in opposite directions, and during growth of a single crystal the rotation rate of the crystal rod is greater than that of the crucible. The rotation rate of the crucible increases as the crystal rod is pulled. A magnetic field is applied to a melt in the crucible so as to provide a component perpendicularly intersecting the bottom and side walls. The intensity of the magnetic field and the component perpendicularly intersecting the bottom and side walls decrease as a single crystal grows. After approximately 50-80% of the fed melt is solidified, application of the magnetic field is stopped. Thereafter, the concentration of oxygen in the single crystal is controlled by increasing the rotation rate of the crucible to the rotation rate of the crystal rod.
This method provides a measure against high oxygen concentration in a top portion of the crystal that is difficult to attain through a method not involving application of a magnetic field, and improves initial oxygen concentration and the distribution thereof. However, effect on prevention of dislocations through application of a magnetic field is poor, since the intensity of the magnetic field decreases and eventually reaches zero. Therefore, the yield of a product is lowered by dislocations, and an effect for greatly prolonging the service life of a crucible, by preventing deterioration of a quartz crucible, is not fully attained. In addition, disadvantageously, the radial distribution of oxygen is not uniformly regulated, since the rotation rate of the crucible increases as the single crystal is pulled to thereby cause a considerable decrease in oxygen concentration at the periphery of the crystal rod.
Japanese Patent Application Laid-Open (kokai) No. 7-61893 discloses a method for growing a single crystal having a uniform distribution of oxygen in which the intensity of the magnetic field applied during crystal growth is regulated by setting a magnet number. In this case, the convection in a melt is regulated to an axially-symmetrical flow without the convection being completed. Briefly, mass transfer in the melt contained in a crucible is not in a diffusion-control mode which is realized by the convention-suppressing effect of a magnetic field, but rather a "controlled convection" is formed without development of vortexes. As a result, oxygen is uniformly incorporated in the growth and radial directions, to thereby produce a silicon single crystal having a uniform oxygen distribution with respect to all directions.
In order to confirm the effects of the above-disclosed method for growing a single crystal, the present inventors have conducted growth of an 8-inch silicon single crystal in a hot-zone by use of a quartz crucible having a diameter of 560 mm. However, no actual effects have been confirmed. In other words, conditions for growth employed in the above-disclosed method are established under very specific restrictions, such as use of a small-diameter crucible. In addition, measurement of convection in a melt employed in this method is considerably complex and requires a great deal of work in consideration of the actual operation. Thus, the method disclosed in Japanese Patent Application Laid-Open (kokai) No. 7-61893 cannot be employed as a generally accepted method for producing a single crystal.